As one of the processes for production of ultrafine metal particles having very small particle diameters, or ultrafine metal particles having an average particle diameter of 100 nm or smaller, Japanese Patent Application Laid-Open No. 34211/1991 discloses ultrafine metal particles with particle diameters of not larger than 10 nm which are prepared by using a gas evaporation method and dispersed in a colloidal state, and a process for production thereof. Further, Japanese Patent Application Laid-Open No. 319538/1999 for example, discloses ultrafine metal particles with an average particle diameter of about several nanometers to several tens of nanometers which are dispersed in a colloidal state by using a reduction precipitation method with an amine compound for reduction, and a process for production thereof.
The ultrafine metal particles with an average particle diameter of about several nanometers to several tens of nanometers disclosed in Japanese Patent Application Laid-Open No. 319538/1999, for example, such that their surface are coated with a polymer resin, for example, so as to maintain the colloidal state.
In general, the ultrafine metal particles with an average particle diameter of about several nanometers to several tens of nanometers are known to be easily sintered at a temperature which is much lower than their melting point (for example, at a temperature of 200° C. or lower in the case of ultrafine silver particles having clean surfaces). The reason is as follows. That is, as the particle diameters of the ultrafine metal particles are reduced to a sufficient extent, a proportion of atoms in a state of high energy becomes large with respect to total of the atoms present on the surfaces of the particles and the surface diffusion of the metal atoms becomes considerably high, with the result that interfaces between the particles are expanded due to the surface diffusion and thus the particles are sintered.
To attain such an extensive sinter as to achieve a desired performance in conductivity, the polymer resin coating the surfaces needs to be thermally decomposed or evaporated, and the temperature for the treatment must be set higher than at least 300° C., even when silver which has a low melting point is used as the conductive material.
The above surface diffusion itself in the ultrafine particles occurs even at temperatures lower than 300° C. Thus, when the contained ultrafine particles together form compact sintered random chains and thereby form a network as a whole in order to eventually attain desired electric conductivity, the polymer resin present on the surfaces of the particles for retaining the colloidal state interferes with the formation of the random chains. Therefore, when drying and curing are carried out at a temperature lower than 300° C., an excessively stabilized colloidal state of the ultrafine metal particles in a paste causes an adverse effect, whereby the particles cannot form the random chains required for conductivity. As a result, resistance becomes too higher for practical use.
However, when it is necessary that sintering be conducted at a temperature higher than at least 300° C., it is needless to say that a substrate on which such a conductive metal paste is printed must be one having a sufficient heat-resistance at a sintering temperature exceeding 300° C. That is, a range of materials usable as a material of the substrate is limited accordingly. In addition, when sintering needs to be conducted at such a temperature, there is no effective organic binder with excellent heat resistance available for the conductive metal paste, and therefore, there remain problems in mechanical strength, i.e., its sinter shows low strength of adhesion to the substrates, and it comes off or is flawed.
Further, once the sintering of ultrafine particles having an average particle diameter of not larger than 10 nm is initiated, following the surface diffusion therein progressing fast, mutual coalescence of ultrafine particles in contact with one another occurs, and also mutual fusion thereof eventually occurs, causing a phenomenon referred to as “particle growth”. The particle growth means a phenomenon that a plurality of fine particles in contact with one another fuse interfaces thereof and integrate one another so as to grow into one big granule. In that case, a reduction in the total surface areas of a plurality of the fine particles as a whole occurs, and gap spaces present between the fine particles included within an envelope of a composite formed by a plurality of the fine particles are eliminated, so that it results in “volume shrinkage” which is a reduction in the apparent volume of the composite. In general, in a conductive metal paste using superfine particles of conventional composition, as an average particle diameter thereof is decreased in size, a ratio of change in a reduction in surface areas and “volume shrinkage” are increased, and cracks may be caused to occur on the surface of a cured product or breakage of a tight-connecting interface between the sinter and the substrate may occur. Due to such reasons, in the conductive metal paste using superfine particles of conventional composition, difficulty of uniform film formation increases along with an increase in film thickness. For example, in formation of a film having a thickness of at least several microns, particularly a thickness exceeding 10 microns, difficulty in controlling its conductivity within a desired range is further increased.
Meanwhile, a commonly used conductive paste for general-purpose uses metal powders with an average particle diameter of 0.5 to 20 μm which are prepared by a grinding method, an electrolysis method, a reduction method or the like, and since the metal powders are physically contacted with one another through curing and shrinkage of a binder resin so as to attain electric conductivity, resistance in each metal particle is sufficiently small, a contact area thereof is also relatively large, and conductivity is also good. On account of these advantages, in a variety of fields where a thickness and line width to be formed is not extremely small, a conventional paste using metal powders having an average particle diameter of 0.5 μm or larger is widely used. However, it is the current situation that as the thickness and line width of a film to be formed decrease along with a decreased wiring pitch in printed wiring and an increased fineness of a circuit in a semiconductor device owing to down-sizing of an information terminal in recent years, the conventional paste cannot conform to such needs since the particle diameters of the metal powders are too large. To state more specifically, for example, when assuming a film having a thickness of about several microns, only two or three metal particles exist in the thickness direction thereof, and in such condition, the intrinsic non-uniformity in contact between particles brings about relatively large deviation in conductivity and consequently, stability in its continuity becomes unsatisfactory. Further, as it composes the decreased number of particles, it shows roughness on the surface reflecting the shapes of the metal particles, thereby impairing the smoothness of the surface.